U.S. helicopters and MV-22 Ospreys are on the ground in northern Iraq and operating from a secure airfield protected by Kurdish forces, the latest sign of an expanding Iraq mission that now includes about 1,000 U.S. troops, defense officials said Wednesday.

The Ospreys arrived overnight Tuesday as they ferried about 130 additional U.S. troops into the airfield, which is in or near the Kurdish city of Irbil, said Army Col. Steve Warren, a Pentagon spokesman.

“They will stay there until they are no longer required,” Warren said of the aircraft. “There is no timeline.”

The new U.S. troops include more than 80 Marines and dozens of special operations soldiers. Those troops were deployed from other locations within U.S. Central Command, Warren said.

The expanding Iraq operation, which has no assigned name, comes as the White House is considering a more aggressive humanitarian mission to rescue the estimated 40,000 Iraqi Yazidis who are trapped on a nearby mountain, surrounded by Islamic State militants and on the verge of starvation.

The precise number of U.S. aircraft on the ground in northern Iraq is likely to fluctuate, but for now includes about four MV-22 Ospreys and several rotary-wing aircraft, defense officials said.

The aircraft will support the 130 new U.S. troops authorized by President Obama on Tuesday night. Their mission is limited to conducting intelligence assessments on Islamic State forces and helping to prepare possible recommendations for an expanded humanitarian assistance mission to help the Yazidis. Among those 130 new troops are some aircrew personnel and maintainers, Warren said.

About 1,000 U.S. troops are now in Iraq, including about 200 in Irbil, which is under assault from Islamic State forces. In total, about 900 troops are providing security for U.S. personnel and conducting intelligence assessments. About 100 more are on permanent assignment to the U.S. Office of Security Cooperation-Iraq and arrived before the militants began seizing large swaths of Iraq territory in June.

On Wednesday, a White House official reiterated Obama’s vow to keep U.S. troops out of direct combat. “What he’s ruled out is reintroducing U.S. forces into combat on the ground in Iraq,” said Ben Rhodes, a deputy national security adviser.

However, the precise definition of “combat” is unclear. At the Pentagon, Warren said he did not want to get into “the whole debate over, ‘When does an action become combat?’ ”

“Make no mistake ... these personnel are in Iraq where there is unrest and there is an active enemy. That said, their purpose is [to] assess ... options for humanitarian assistance [in Irbil]. In Baghdad, their purpose is to assess the capability of the Iraqi security forces,” Warren said.